Are these slow-burning beasts the key to forecasting quakes?

The East Coast's Mahia Peninsula has been a hot-spot for slow slip quakes. Photo / NZ Herald

Scientists are about to get an unprecedented look at the one of the largest "slow-slip" earthquakes ever observed in New Zealand – and what they learn could help forecast future shakes.

Only discovered in recent times, slow slip events are silent, slow-burning earthquakes that can displace faults over days or months.

By contrast, in a typical earthquake, fault movement occurs over a matter of seconds, unleashing an instant surge of energy.

Slow quakes have been found to unfold particularly around the sprawling plate boundary to the east of the country that is the Hikurangi Subduction Zone.

Scientists have watched them play out every one to two years near Gisborne, at a relatively shallow depth beneath the seabed, and usually driving a spate of localised quake activity.

The most recent one, which began around early April and last several weeks, triggered a swarm of East Coast quakes - among them a magnitude 5.1 jolt that struck near Mahia in May.

In nearly two decades of observations, that event was considered one of the biggest on record in New Zealand.

It also happened at the very time GNS Science researchers had set an array of sophisticated instruments on the seafloor, from north of Gisborne to the south of Hawke's Bay.

"The really exciting thing about this event is it's the first time we've had so many different types of observations going at once, right on top of the slow slip," GNS geophysicist Dr Laura Wallace said.

"Assuming all the instruments are working properly, we should be able to transform our understanding of where these slow slip events are happening offshore the east coast, particularly off Hawke's Bay."

'Slow-Slip Central'

To understand the significance of these mysterious occurences is to understand the massive system that they happen in.

The Hikurangi Subduction Zone is a major offshore fault is where the Pacific Plate dives - or subducts - westward beneath the North Island. Image / GNS Science

The Hikurangi Subduction Zone is a major offshore fault is where the Pacific Plate dives – or subducts – westward beneath the North Island, and poses New Zealand's largest geological hazard.

Scientists believe the zone has the potential to unleash "mega-thrust" earthquakes larger than magnitude 8, such as those which created tsunamis that devastated Indonesia in 2004 and Japan in 2011.

Over the past decade, the system has been at the centre of a massive scientific collaboration between the world's leading earthquake experts, involving over $60m of international investment.

Already, scientists have put in place two sub-seafloor earthquake observatories - making New Zealand only the fourth country in the world to have such capabilities.

The technology could help pave the way for offshore instrumentation needed for earthquake and tsunami early warning systems.

Scientists regard the region as one of the best places in the world to study slow slip events, leading them to set up a range of state-of-the-art sensors for that specific purpose.

The black arrows show the horizontal displacement of the GeoNet GPS sites over the last week, while the yellow to red colors show the areas of the Hikurangi Subduction Zone fault that have been slipping. There has been up to 10 to 15 cm of movement on the plate boundary offshore Gisborne in the last week. Source / GNS Science

In November, Wallace and colleagues plan to head out to sea to retrieve data from ocean bottom seismometers - eventually enabling them to pin-point precisely where the quakes that came with the slow slip event struck - and sea floor pressure sensors.

"These seafloor pressure sensors are right on top of where the slow slip event happened, and are sensitive to small upward or downward movements of the seafloor – and we hope to be able to detect on the order of a few centimetres of upward movement above the slow slip areas," she said.

Some of the quakes recorded off the coast of the Mahia Peninsula during a recent slow-slip event - including a 5.1 quake, shown here as the largest dot. Image / GeoNet

"They're essentially going to give us a much clearer picture of the location and amount of slow slip that occurred offshore. At the moment, we have no idea what this might be because the data from GeoNet that we currently have to go on is only from land-based instruments."

Quake mystery

So why did the slow slip events actually happen?

Wallace said they often relieved stress that had built up along the plate boundary.

"But they can also distribute stress to other areas of the plate boundary, and can thereby trigger swarms of small earthquakes," she said.

This graphic shows the locations of the quakes in an April swarm and their magnitudes. Image / GNS Science

"That is something critically important that we need to get a handle on, because if we can better understand the influence of slow slip events on earthquake occurrence, we can eventually use that to do better near-term earthquake forecasting.

"We'll probably never be able to actually predict earthquakes, but slow slip events and their relationship to earthquakes really offer a lot of hope for doing better forecasting."

She clarified, however, that the events shouldn't be seen as warning signals of mega thrust quakes and tsunamis.

"There have been some cases of slow slip preceding large earthquakes – like Tohoku in Japan in 2011 – but when that does happen, it's rare, so people shouldn't freak out whenever a slow slip happens. We've seen hundreds of slow slip events that don't lead to large earthquakes."

Ultimately, she was fascinated to learn what wider effect the latest event might have had1 on the subduction zone.

"Right now, we are collecting a whole variety of information relating to contortion of the Earth's crust near the subduction zone, and what's happening with the flow of fluids within the region."

"Slow slip events and their relationship to earthquakes really offer a lot of hope for doing better forecasting," GNS scientist Dr Laura Wallace says. Photo / Supplied

A recent paper by GNS colleague Dr Emily Warren-Smith, involving a detailed analysis of several hundred earthquakes between Hawke's Bay and East Cape, had already offered the first physical evidence of the way stresses changed before, during and after slow-slip events.

Image / GNS Science

Her study concluded that highly pressurised fluid released from the subducting plate travelled upward and lubricated the interface between the two plates.

This initiated a slow-slip event where a large patch of the fault moved slowly and benignly for weeks or months and then stopped, acting to temporarily relieve pressure before the process started over again.

These observations had been taken from a 2014-15 slow slip event, and Wallace expected the fresh data about to be collected from the recent, larger event an even clearer picture of this process for Warren-Smith to investigate.